Technical Field
The present invention relates to a method of controlling a gasifier. Particularly, the invention relates to controlling the temperature of a circulating fluidized bed gasifier gasifying uniform fuel, especially, plenty of volatile matter containing fuel, such as forest residue or other biomass, having a mixture of oxygen and steam as a gasification agent.
Background Art
A circulating fluidized bed gasifier having a mixture of oxygen and steam as a gasification agent can advantageously be used for gasifying, for example, biofuels. It is generally known that a high temperature in the lower portions of the bed increases risk of agglomeration of the bed. On the other hand, a low temperature in the upper portions of the gasifier may leave a relatively high amount of tars in the product gas, which decreases the gasifying efficiency and may cause fouling of the gas coolers and filtration units.
U.S. Pat. No. 6,117,199 discloses a typical circulating fluidized bed gasifier in which oxygen-containing gas is introduced as fluidizing gas through a bottom grid. Material to be gasified is fed to the gasifier 3 to 6 meters above the bottom grid to be converted into product gas and residual carbon in a reducing zone in the upper portions of the gasifier. Residual carbon is separated from the product gas and returned to the gasifier to be oxidized in an oxidizing zone in the lower portion of the gasifier. The circulation of bed material distributes the heat generated in the oxidizing reactions so that the temperature is 970 to 1200° C. in the oxidizing zone and 70 to 120° C. lower in the upper portions of the gasifier.
U.S. Pat. No. 4,400,181 discloses a pressurized circulating fluidized bed gasifier using a mixture of steam and oxygen as a gasifying agent for gasifying coal, which is fed into the gasifier at the center portion of the gasifying reactor. The temperature profile is controlled by feeding the gasifying agent into the gasifier at two levels, and by returning separated primary char to the center portion of the gasifier and finer secondary char to a lower portion of the gasifier.
U.S. Pat. No. 4,968,325 discloses a pressurized bubbling bed gasifier for gasifying biomass by steam-free oxygen-containing gas, for example, oxygen. The oxygen-containing gas is fed into a bottom section of the gasifier and the biomass is introduced into the area of maximum velocity, at a center portion of the bed, so as to obtain efficient distribution of the biomass into the bed.
U.S. Patent Application Publication No. 2010/0146856 A1 discloses feeding high-calorific-value feedstock, such as coal, with an oxidant into a first gasification zone and a low-calorific-value, high oxygen content feedstock, such as biomass, into a second gasification zone of a gasifier so as to control temperature distribution in the gasifier.
Japanese patent publication No. 57-115489 A discloses a fluidized bed gasifier in which a coarse portion of powdered coal is fed into the bottom of a fluidized gasifying bed and a fine portion of the powdered coal is fed to a top enlarged part of the gasifier. An advantage of the process is that generated tars are catalytically decomposed and gasified in the top portion of the gasifier.
It has also been suggested that the temperature profile of a circulating fluidized bed gasifier having a mixture of oxygen and steam as a gasification agent can be controlled by introducing fluidizing gas with a relatively low oxygen content, such as recirculated product gas, through a bottom grid of the gasification chamber, and introducing a controlled amount of a mixture of oxygen and steam as a secondary gas, and possibly, also as a tertiary gas, into higher portions of the gasification chamber. According to recent model-calculations having a mixture of oxygen and steam as a grid fluidizing gas, however, feeding of reasonable amounts of additional oxygen and steam as secondary and tertiary gas higher up in the gasification chamber does not considerably change the temperature profile, but the main effect of the additional gas feeding is that the average temperature in the gasifier increases. In any case, the use of recirculated product gas as a fluidizing gas requires equipment for recirculating the gas and brings about safety issues related to conveying and handling of combustible product gas. Furthermore, the use of high amounts of secondary and tertiary gas with high-oxygen content increases the risk of local hot spots and sintering in the upper portion of the gasifier.
Because of the need to minimize the tar concentration in the product gas, it is important that the temperature in the upper portion of the gasifier is sufficiently high, for example, 870 to 900° C. In case the temperature in the lower portion of the gasifier is clearly higher than that in the upper portion of the gasifier, maintaining a sufficient temperature in the upper portion may result in a too high temperature in the lower portion of the gasifier and cause operational problems, especially, agglomeration of the bed. A relatively large temperature difference between the upper and lower portions of a fluidized bed gasifier appears to prevail, especially in high operating pressures of the gasifier and with high oxygen contents of the fluidizing gas. Therefore, it is desirable to find new ways to simultaneously increase the temperature in the upper portions of the gasifier and lower the bed temperature so as to have an as uniform temperature profile as possible.
Another attempt to unify the temperature profile of a circulating fluidized bed gasifier includes arranging a heat exchanger for cooling circulating particles before they are introduced into the bed. However, according to model calculations, the cooling of the circulating particles surprisingly does not cool the bed locally so as to affect the temperature profile in the gasifier, but the profile remains approximately unchanged, and only the average temperature of the gasifier decreases.
An object of the present invention is to provide a simple and an efficient method of controlling the temperature distribution in a circulating fluidized bed gasifier.